Nature of operations and summary of significant accounting policies Nature of Operations and Summary of Significant Accounting Policies--Oil and Natural Gas Properties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capital costs for undeveloped acreage	$ 190,356	$ 196,227
CO2 Delivery Pipelines and Facilities [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exploration and development costs excluded from amortization	72,046	115,828
Wells and Facilities in Progress [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exploration and development costs excluded from amortization	$ 26,023	$ 9,422